Semiconductor devices and liquid crystal display devices are manufactured by a so-called photolithographic method, in which a pattern formed on a mask is transferred onto a photosensitive substrate. An exposure apparatus used in the photolithographic process has a mask stage that supports a mask, and a substrate stage that supports a substrate, and it transfer a pattern on the mask onto the substrate via a projection optical system, while sequentially moving the mask stage and the substrate stage. Recently, higher resolution is desired for the projection optical system, in order to address higher integration of device patterns. The resolution of the projection optical system increases as an exposure wavelength to be used becomes shorter and a numerical aperture of the projection optical system becomes larger. Therefore, the exposure wavelength used in the exposure apparatus becomes shorter year after year, and the numerical aperture increases as well. The exposure wavelength, which is dominantly used at present, is 248 nm of a KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used.
When the exposure is performed, depth of focus (DOF) is important as well as the resolution. The resolution R and the depth of focus δ are respectively expressed by the following expressions:R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
Here, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represents process coefficients. From the expressions (1) and (2), it is seen that if the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ becomes narrow.
If the depth of focus δ becomes too narrow, it is difficult to align the substrate with respect to the image surface of the projection optical system, and a focus margin during the exposure operation may be insufficient. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in Patent Document 1 described below as a method for substantially shortening the exposure wavelength and widening the depth of focus. According to this liquid immersion method, the space between a bottom surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent to form a liquid immersion area, to improve the resolution and, at the same time, enlarge the depth of focus by approximately n times by taking advantage of the fact that the wavelength of the exposure light in the liquid becomes 1/n times that in the air (n represents the refractive index of the liquid, and is generally about 1.2 to 1.6).    Patent Document 1: PCT International Publication No. WO99/49504